Three-dimensional integrated circuits are employed in applications in which space is a critical design factor. As the demand for more functionality in less space increases, so does the number of designs using three-dimensional packaging. In addition to the benefit of reducing space, these designs may also realize higher speeds because interconnects between circuit components may be shorter.
Memory stacking was the first application of three-dimensional packaging, but now applications range from stacking memory modules to stacking entire systems. Different layers in the stack may have different functionalities. For example, one layer may be a memory layer and another may be a logic layer. It is also possible that the different layers in the stack could have different dimensions.
These applications may require the precise stacking of very thin platelets into cubes. Platelets may consist of a semiconductor chip placed in a chip carrier. The platelets themselves may be less than 5 mils thick and there may be as many as sixty platelets stacked in one cube. It is critical that the spacing between the platelets is held to a very tight tolerance and that the platelets are not damaged during the stacking procedure.
Typically, the platelets are held in the cubical stack by a very thin layer of epoxy resin between each layer. This epoxy layer may be less than one micron thick. Other bonding materials, such as silicone rubber or eutectic solder alloy, may also be employed. The required spacing dimensions may be maintained by using an apparatus with a calibrated compression arm that applies pressure to the stack while the epoxy is setting. A typical amount of pressure may be ten Newtons of force. With this arrangement, only the overall cube dimensions can be maintained, and great care must be taken to prevent cracking the delicate platelets by excessive pressure. The critical layer to layer spacing is thus a derived property and is based upon the uniformity of the thickness and pressure-flow characteristics of the adhesive layer.
It would be desirable to provide a stacking method that provides a very tight spacing tolerance between the platelets and that minimizes damage to the platelets during the stacking process. The invention addresses current limitations and makes the critical spacing a directly controlled property resulting in much higher accuracy potential with a relative independence of the adhesive layers.